1. Field of the Invention
The invention relates to electromagnets generally and more specifically to electromagnets used to manipulate magnetic fields in enclosed vessels.
2. Description of Related Art
Magnetic fields have long been used to manipulate particles that respond to them such as charged particles or polar particles. In one large class of such uses, electromagnets located outside a vacuum vessel or reactor vessel are used to affect the behavior of particles that respond to the magnetic fields within the vessel. Examples of such uses are focusing electromagnets in cathode ray tubes or particle accelerators and electromagnets used to manipulate plasmas in plasma reactors or to control crystal growing processes.
An exemplary use of electromagnets to manipulate plasmas is magnetic enhanced reactive ion etching, or MERIE. Part of the process of producing an integrated circuit is etching the substrate for the circuit. A common way of doing this is by reactive ion etching, in which reactive ions do the etching. The parts of the substrate which are not to be etched are covered by a mask which is resistant to the reactive ions and the masked substrate is placed in a vessel in which a plasma of reactive ions is formed from a reactive gas. The reactive ions react with the unmasked areas of the substrate and the reaction etches the substrate. In MERIE, a magnetic field is used to manipulate the plasma to keep the plasma within the area over the substrate and increase the plasma's density, which increases the rate of etching. For details on MERIE and the reactors generally used in MERIE, see the Schlueter patent cited above.
One approach to manipulating the plasma in a MERIE reactor is simply to generate a constant uniform magnetic field in the vessel that keeps the plasma where it is wanted. One way of doing this employs the principles of the Helmholtz coil. A Helmholtz coil consists of two plane circular coils of radius r whose planes are parallel to each other. The centers of the coils are on a line perpendicular to the planes and the planes are r apart. In the area between the coils, the combined fields of the coils produce a generally constant magnetic field. An example of the use of the principles of the Helmholtz coil in a chemical vapor deposition reactor may be found in U.S. Pat. No. 4,668,365, Foster et al., Apparatus and method for magnetron-enhanced plasma-assisted chemical vapor deposition, issued May 26, 1987, in which a pair of electromagnetic coils having the geometry of the Helmholtz coil are used to provide a uniform magnetic field in which material is deposited on a substrate that is perpendicular to the planes of the Helmholtz coil. An example of the use of pairs of electromagnetic coils having the geometry of the Helmholtz coil to confine the plasma in a reactor vessel to the area between the electrodes of the vessel may be found in U.S. Pat. No. 5,527,394, Heinrich, et al., Apparatus for plasma enhanced processing of substrates, issued Jun. 18, 1996.
Another way of producing a more uniform magnetic field is described in U.S. Pat. No. 5,718,795, Plavidal, et al., Radial magnetic field enhancement for plasma processing, issued Feb. 17, 1998. Here, the magnet which produces the field is located above the ceiling of the vessel. The magnet has a ferrous yoke which has a hub connected by spokes to an outer ring. The hub, spokes, and ring form a shallow cone. The spokes have windings which are connected to an AC, DC, or RF power supply. The angle of the spokes is chosen to produce a relatively uniform radially symmetrical magnetic field at the level of the substrate.
A way of dealing with non-uniform plasmas is to use the electromagnets not only to keep the plasma where it is wanted, but to move the plasma. The most common arrangement for doing this is described in the Background Art portion of the Schlueter patent. Four electromagnetic coils are mounted 90° from each other on the sides of the vessel. The coils have vertical legs and horizontal legs, with the horizontal legs sometimes being bent to conform to the curve of the sides of the vessel. While the magnetic field produced by an opposed pair of the electromagnetic coils is not particularly uniform, the field can be rotated by varying the amount and direction of current in pairs of opposite coils. Rotation may be achieved by switching the current in the coils or more smoothly by applying AC currents to the coils in the amounts, phases and frequencies required to rotate the magnetic field. Slowly rotating the plasma in this fashion reduces the exposure of the circuitry in the substrate to irregularities in the plasma which can damage the circuitry.
Though the electromagnetic coils on the sides of the vessel permit rotation of the magnetic field, the lack of uniformity of the magnetic field both causes irregularities in the plasma and these in turn may lead to damage in the substrate. One way of producing a more uniform rotating magnetic field is described in the Schlueter patent; there, the magnetic field is generated by a number of coils wound on a ferrous cylinder that surrounds the outside of the vessel that contains the substrate at the position of the substrate. On the inside of the ferrous cylinder, the windings are parallel to the vertical axis of the vessel. The main function of the ferrous cylinder is to shield the windings on the inside of the ferrous cylinder from the windings on the outside of the cylinder. The windings on the inside of the ferrous cylinder generate a magnetic field in the area of the substrate. The field can be varied by varying the amounts of current provided to the different coils and the field can be rotated by varying the amounts and directions of the current in the coils according to a periodic pattern.
None of the foregoing techniques for producing magnetic fields inside a reactor vessel provides a perfect solution for MERIE. Ideally, it should be possible not only to produce a uniform magnetic field but also to produce a magnetic field with controlled gradations across the plane of the substrate and/or along the vertical axis of the vessel, and it should be possible to rotate or otherwise move whatever field is produced. It is further desirable to be able to retrofit current reactors employing coils on the sides of the vessels to produce rotating magnetic fields with electromagnets that are capable of producing highly-uniform rotating magnetic fields without affecting the retrofitted reactor's power supplies or arrangements for inserting and removing substrates or providing gaseous inputs.
It is thus an object of the invention disclosed herein to provide electromagnets which are capable of producing uniform magnetic fields, magnetic fields with controlled gradations in either the horizontal or vertical directions, and magnetic fields which can be rotated and moved and which can further be retrofitted to existing reactor vessels that employ coils on the sides of the vessels.